1. Field of the Invention
The present invention relates generally to optical fiber (hereinafter xe2x80x9cfiberxe2x80x9d) and more particularly to a multiple crucible method of making fiber.
2. Technical Background
The most widely known multiple crucible method of drawing fiber is the double crucible method. The double crucible method of making fiber has been known for at least the last two or three decades, and is disclosed for example in Optical Fibers for Transmission, New York John Wiley, pp. 166-178 (Midwinter, 1979).
Much of the early work in the area of drawing multimode fiber by double crucible exploited the natural tendency of the fiber core and cladding to diffuse (mix). This type of fiber is typically characterized as a fiber with a large core relative to the cladding and a less than step function refractive index change at the interface of the core and cladding (hereinafter multimode fiber). Later work in this area was directed toward creating a parabolic index profile in the multimode fiber. This was accomplished by controlling the diffusion between the core and the cladding.
In the case of making single mode fiber from a double crucible, the challenge of limiting diffusion is much greater for at least the reason that the core of a single mode fiber is much smaller than the core of a multimode fiber. Typical core size of a single mode fiber is 3-6 microns. Thus a diffusion depth of 1 micron may affect a significant portion of the core of a single mode fiber.
Prior attempts to limit diffusion in a single mode fiber have been made in the rod-in-tube method of manufacturing fiber. This method requires the use of distinct portions of core and cladding material. The distinct portions of core material are inserted into the cladding material. This assembly is then consolidated and drawn at a high viscosity. This method suffers from interfacial contamination, increased fiber attenuation, and additional thermal histories being imposed on the glass. The application of additional thermal history is especially harmful to types of glasses that are prone to devitrification.
Another challenge in drawing a single mode fiber from a multiple crucible is the core size. If the core size is too large, the fiber will not be single mode. If the core size is too small, the fiber will not support a mode of light at the required wavelength. Another issue is maintaining the consistency of the diameter of the core along the length of the fiber.
Past attempts to draw a fiber with a consistent suitably sized core have focused on the issues of the viscosity of the glass raw materials, the ratio of orifice diameter of the crucibles for the core and cladding glasses, as well as the draw speed. However, these attempts have not demonstrated the necessary sensitivity to be applicable to the requirements for the core size of a single mode fiber.
The present invention relates to a method of making an optical fiber drawn from a multiple crucible. The method includes moving a first crucible of the multiple crucible relative to a second crucible of the multiple crucible. An optical fiber is drawn from the multiple crucible and the location of the core of the fiber with respect to the cladding of the drawn fiber is documented. The aforementioned steps of moving the first crucible, drawing fiber, and documenting the location of the core of the drawn fiber are repeated until the core is located at a desired point of the drawn optical fiber. One example of a desired point of the drawn fiber is the center of the fiber.
One embodiment of the invention includes establishing a position of the core of an optical fiber drawn from the multiple crucible, preferably the position is a radial position. This can be facilitated by employing a graduated x-y plane along a cross section of the fiber, along which the location of the core of the fiber can be moved. The location of the core, of fiber drawn subsequently to the employing step, along an x-axis of the x-y plane is altered to locate the core at a center of the x-axis. The location of the core, of fiber drawn subsequently to the employing step, along a y-axis of the x-y plane is altered to locate the core at a center of the y-axis. Advantages which result from practicing the method disclosed herein include the ability to center the core of the fiber irrespective of the circumferential orientation of the core of the fiber.
A further invention described herein is a method of making fiber from a multiple crucible apparatus to minimize core and cladding diffusion. A tip of the first crucible is aligned axially above a tip of the second crucible by a preselected distance to minimize core and cladding diffusion. A fiber is drawn from the multiple crucible apparatus under non-isothermal conditions of a temperature gradient of least about 10xc2x0 C. per mm. The temperature gradient extends from at least the tip of the first crucible to the tip of the second crucible. By orienting the tips of the two crucibles appropriately with respect to one another, the diffusion between the core and cladding of a fiber drawn by a multiple crucible method is minimized, thereby enabling the drawing of fiber having a step refractive index change at the core and clad interface. Furthermore, practicing the invention enables maintaining the core/clad interface cleanliness without imposing additional thermal histories on the glass. A still further advantage which may result from practicing the above method is minimizing the potential of devitrification occurring in the drawn fiber. In turn, minimizing devitrification reduces attenuation and improves the tensile strength of the drawn fiber.
An additional method described herein includes applying a differential pressure to a body of raw materials in the first crucible. The application of the pressure effects a change in the size of the diameter of the core of the drawn fiber. One advantage of the aforementioned method includes the ability to adjust the size of the core of the fiber to a predetermined size. This advantage includes the ability to continually draw the fiber and maintain the core diameter within a predetermined range. Furthermore, practicing the above method enables the production of an optical fiber having an accurate core geometry along the length of the fiber.
All of the above methods include the advantage of being applicable to both soft and hard glasses. The above methods also have in common at least one more advantage, that the methods may be easily incorporated into existing fiber draw equipment.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.